X-ray inspection systems are often used to inspect objects that may be difficult to inspect using optical or other inspection techniques. For example, x-ray inspection systems are particularly useful in the inspection of objects that are embedded within, or are otherwise visually blocked by, other objects. X-ray inspection involves the capture of projected images of an object under inspection by one or more x-ray sensors. In this regard, one or more x-ray sources generate x-rays that may illuminate one or more sensors as attenuated by an intervening object under inspection. During image acquisition, the quality of the images captured by the one or more sensors may be limited due to the presence of x-ray scatter, which can result in loss of dynamic range in a captured image, thus reducing the system's inspection capability.
To reduce interference from x-ray scatter, a collimator may be placed between an x-ray source and the object space. As used herein, the term “collimator” refers to a device that produces directed beams from one or more x-ray sources. For example, a collimator may collimate x-rays from one or more x-ray sources into one or more fan beams. As used herein, the term “fan beam” refers to an x-ray beam having a constant ratio of major to minor dimension at any transverse cross section. To reduce x-ray scatter in an X-ray inspection system, a collimator may be used to collimate the x-rays generated by the x-ray source(s) into a number of fan beams, each directed at a respective sensor along a respective controlled solid angle. For example, a collimator may be configured to produce one or more fan beams having respective ratios of major to minor dimensions that substantially match respective ratios of major to minor dimensions of corresponding sensor areas—thus, the x-rays substantially illuminate only the sensing area of the sensor(s) located in the imaging plane.
To obtain the advantages of X-ray collimation, very precise tapered windows must be machined into material that is an effective attenuator of X-rays. Tungsten (W) is generally the material of choice for an application where precision, strength and X-ray attenuation are required, although any x-ray attenuating material may be used. However, the manufacture of Tungsten collimators is very expensive due to the miniscule complex shapes that must be created. Although Tungsten is quite pliant in its purest form, it typically contains small concentrations of carbon and oxygen, which gives tungsten metal its considerable hardness and brittleness. Given these properties, collimators made from Tungsten or other hard materials are typically created using Electrical Discharge Machining (EDM) techniques.
In EDM metal is removed by producing a rapid series of repetitive electrical discharges between an electrode and a metal workpiece. The electrical discharges actually remove small amounts of material and allow the electrode to be moved through the metal. The path of the electrode is typically controlled by a computer, which allows extremely intricate contours or delicate cavities that would be difficult to produce with a grinder or other cutting tools.
Lead (Pb) and other softer X-ray attenuating materials can also be used to create lower cost X-ray collimators, but the window sizes must be increased and additional thickness is required compared to Tungsten collimators. Larger windows make a collimator less effective at attenuating scatter X-rays and the additional thickness required for equivalent attenuation reduces the allowable distance from the X-ray source to the object being imaged.
Accordingly, a need exists for creating complex shaped 3-dimensional internal structures in hard materials such as Tungsten to allow the fabrication of effective X-ray collimators at a cost much lower than traditional methods of construction.